Avian muscular dystrophy is characterized by the degeneration of fast white skeletal muscle fibers, with onset during development. Results of histological studies and observations on certain contractile proteins (myosin light chain-3, troponin-T and tropomyosin) have suggested that the disease may involve the persistence of the embryonic fiber type. Our recent observations during development in normal and dystrophic birds on myosin heavy chain isozymes, which are a major factor in histological fiber typing, indicate that the situation is more subtle than this. During development in the normal bird, evidence is emerging from peptide-mapping techniques for the sequential expression of at least four myosin isozymes, detectable at 12-days in ovo (embryonic), 4-days post-hatch (neonatal), 25-days post-hatch (prepubescent) and at some point subsequent to 83-days post-hatch (adult). The dystrophic bird expresses isozymes identical to the normal at 12-days in ovo and 4-days post-hatch. In the adult a dystrohy-specific heavy chain is present differing in sequence from that of the normal bird. Very preliminary data suggests that expression of the prepubescent isozyme may be delayed in dystrophy. Whether the adult dystrophic variant represents a developmental isozyme which fails to give way to the normal adult form or whether it is the expression of a silent or mutant adult gene awaits further work. This proposal aims to complete the age study described above, characterize the various isozymes detected as to sequence and methylated amino acid content, confirm the association of the adult dystrophic variant with dystrophy by carrying out a genetic linkage study and determine whether myosin heavy chain abnormalities exist in murine and human dystrophies.